Since you posted another question dealing with Seleucid, I think the Ptolemy of whom you speak is one of Alexander the Great's generals who divided his empire after Alexander's death. The Empire was divided between Ptolemy, who took Egypt and established that line, Seleucid, who took parts of the old Persian Empire, and Antigone, who took Greece.
Ptolemy established a dynastic line in Egypt which lasted until Egypt was conquered by the Egyptians. Cleopatra was herself a member of the Ptolemaic line, as was her brother, Ptolemy IV. As an interesting side note, scholars had a difficult time translating hieroglyphics until the discovery of the Rosetta Stone. Even then, translation was difficult. The first word actually translated on the stone was the name "Ptolemy." This was the key to the translation of hieroglyphics.
The Ptolemy addressed in the first response was Claudius Ptolemy, the noted Astronomer. He bore no relation to the Ptolemaic line of Egyptian rulers.
There have been more than one important historical figure named Ptolemy. However, since other of your questions have been about figures like Aristotle, I assume that you are asking about the Ptolemy who was an astronomer and mathematician.
Ptolemy lived in Egypt when it was part of the Roman Empire. He is believed to have been born late in the 1st century CE and to have lived into his 70s.
Ptolemy is best known to most people for his geocentric model of the universe. He had figured out that the world was round and he then, using observations of heavenly bodies, proposed that everything else moved in spheres around it. His ideas were taken as fact until 1500s when Copernicus and Kepler proved them wrong.
Ptolemy was also important for his work in trigonometry. His work in that area has stood up better over time in that it has not been proven wrong. However, because astronomy is somewhat more understandable to most people, Ptolemy continues to be best known for his geocentric model of the universe.